Malaria afflicts several hundred million and kills more than 600,000 people each year, mostly children in Sub-Saharan Africa. Aspartic proteases have long been antimalarial targets of interest. A large number of aspartic protease inhibitors that are potent against parasites in culture have been developed, but their specific targets have been elusive. Plasmepsin X (PMX) is one of the least characterized aspartic proteases found in asexual intraerythrocytic malaria parasites. We have recently found that PMX is a key enzyme for intraerythrocytic parasite egress and invasion. It activates the master trigger subtilysin-like protease 1 (SUB1), launching proteolytic events that allow merozoites to get out of the host red blood cell (RBC) and invade fresh RBCs. We have identified a class of aspartic protease inhibitors called aminohydantoins that appear to kill parasites through PMX blockade, preventing SUB1 activation and impairing egress/invasion. PMX knockdown phenocopies inhibitor action. One of the inhibitors has favorable pharmacokinetic properties and gives oral cure in a rodent malaria model. We believe that PMX is an exciting new drug target but need to better characterize its function to inform ongoing drug development and enhance our understanding of parasite biology. To address these questions, aim 1 will examine the specificity of PMX and will address the question of whether SUB1 maturation by PMX is direct. Biochemical assays using isolated PMX with SUB1 as a substrate and with a random peptide library will be performed. Aim 2 will focus on what PMX interacts with. Is SUB1 the only substrate? What else is in the secretory vesicle called the exoneme, where PMX and SUB1 both reside? Aim 3 will address the question of how PMX itself gets activated. Our preliminary data suggest that there must be an upstream enzyme. We will characterize the processing and look for a maturase. We anticipate that the proposed studies will yield great insight into the pathogenesis of malaria and will point the way to new therapies for this devastating disease.